Oscilloscope crossover distortion test evaluates the relationship between oscilloscope sampling rate and sampling fidelity

Although it is difficult to test the sampling distortion of digital clock signals, it can still be done. However, we do not recommend visual distortion testing of digital signals. This is because there is no absolutely "pure" digital clock generator. Even a digital signal generated by a high-performance pulse generator will have varying degrees of overshoot or perturbation and will have different edge speeds. In addition, due to the pulse response characteristics of the oscilloscope and the frequency response that may not be flat, the front-end hardware of the oscilloscope may cause distortion of the pulse waveform of the digitized signal.

However, some tests can be performed using a high-speed clock signal to compare the measurement quality of the oscilloscope ADC system. One such test can compare the stability of parameter measurements, such as the standard deviation of rise and fall times. Cross-sampling distortion will cause unstable edge measurements and add deterministic jitter components to the high-speed edges of digital signals.

Figure 5a: A 400-MHz clock signal captured using a Keysight Infiniium 3-GHz oscilloscope at 40 GSa/s. Figure 5b: A 400-MHz clock signal captured using a Tektronix 2.5-GHz oscilloscope at 40 GSa/s.

Figure 5 shows two oscilloscopes with similar bandwidths capturing and measuring the rise time of a 400 MHz clock signal with edge speeds in the 250 ps range. Figure 5a shows a Keysight 3 GHz bandwidth oscilloscope interleaved with a pair of 20-GSa/s ADCs sampling the signal at 40 GSa/s, resulting in a repetitive rise time measurement with a standard deviation of 3.3 ps. Figure 15b shows an image of a Tektronix 2.5 GHz bandwidth oscilloscope interleaved with four 10-GSa/s ADCs also sampling the signal at 40 GSa/s. In addition to showing more instability, the rise time of this digital signal has a standard deviation of 9.3 ps. The more accurate ADC calibration in the Keysight oscilloscope, combined with the lower noise floor, allows the Keysight oscilloscope to more accurately capture the higher frequency harmonics in this clock signal, providing a more stable measurement.

When using FFT to analyze the frequency components of a digital clock signal, its spectrum is much more complex than testing the spectrum of a simple sine wave. A pure digital clock pulse generated by a high-quality pulse generator should consist of the fundamental frequency component and its odd harmonics. If the duty cycle of the clock pulse is not exactly 50%, the spectrum will also contain low-amplitude even harmonics.

However, if you know what to measure and what to ignore, you can use the oscilloscope's FFT math function to measure the cross-sampling distortion of a digital signal in the frequency domain.

Figure 6a: FFT analysis of a 400-MHz clock signal using a Keysight Technologies Infiniium 3-GHz bandwidth oscilloscope.

Figure 6a shows the spectrum of a 400-MHz clock signal captured with a Keysight 3-GHz bandwidth oscilloscope at 40 GSa/s sampling rate. The only frequency spurs that can be observed are the fundamental frequency component, the third, fifth, and seventh harmonics, and some even harmonics. All other spurious signals in the spectrum are well below the in-band noise floor of the oscilloscope.

Figure 6b: FFT analysis of a 400-MHz clock signal using a Tektronix 2.5-GHz bandwidth oscilloscope

Figure 6b shows the spectrum of a 400-MHz clock signal captured using a Tektronix 2.5-GHz bandwidth oscilloscope, also at 40 GSa/s sampling rate. In this FFT analysis, we can see not only the fundamental frequency component and its associated harmonics, but also several spurious signals at higher frequencies (around 10 GHz to 40 GHz). These spurious signal images are directly related to a poorly calibrated interleaved ADC system.